1. Field of the Invention
The present invention relates to a fixing device, a control method for the same, and an image forming apparatus.
2. Description of the Related Art
In recent years, a scanner for reading image data of a document, a copying machine for printing image data of a document read by a scanner, a printer or a facsimile machine for printing image data that has been input externally, and a so-called multi function peripheral (MFP) including some of these functions have been used. Such an MFP includes a scanning unit for reading images, an engine unit for producing toner images from each of the corresponding images read by the scanning unit on a recording medium, a fixing device (fixing system) for fixing toner images on a recording medium produced by the engine unit by using a fixing roller and a pressing roller.
A fixing device adopting an electromagnetic induction heating system includes a coil known as an exciting coil in which high frequency current flows in order to generate a magnetic flux for inductively heating a heat generating member. With this configuration, because the heat generating member is directly heated, preheating as required in a heat roller fusing system is not required and a temperature of the fixing member is instantaneously risen to a predetermined temperature. As a result, reduction of warm-up time and energy saving can be achieved. Some fixing devices adopting the electromagnetic induction heating system may include a degaussing coil generating a magnetic flux in a direction for degaussing the magnetic flux generated by the exciting coil in addition to the exciting coil.
When an induction heater (the exciting coil) and the degaussing coil are used in combination, the load imposed on the induction heater changes in response to the energization of the degaussing coil. During operation of the induction heater, switching the degaussing coil ON/OFF causes load (impedance) of the induction heater change. However, ON/OFF time periods of the PWM signals for controlling energization of the exciting coil depending on the load have not been set. In other words, minimum and maximum values of output frequencies of a high frequency power source have not been set depending on the ON/OFF state of the degaussing coil. As a result, power beyond a permissible range may be supplied to the induction heater, which may cause a failure of the induction heater.
For example, Japanese Patent Application Laid-open No. 2008-249948 discloses a fixing device for preventing excessive rising of temperature at end portions of a fixing member during passage of a small-sized paper sheet. In the fixing device, during both a time period while a degaussing coil is open and a time period while it is closed, the correlations between an output power value supplied to a resonance circuit from a high frequency power source and an output frequency of a high frequency power source corresponding to the output power value are obtained. Based on the correlations between the output power value and the output frequency f, the fixing device controls for variably setting the output frequency f of the high frequency power source so that a temperature Ta of the fixing member detected by a temperature detecting section matches a predetermined target temperature Tt. In Japanese Patent Application Laid-open No. 2008-249948, however, no method for preventing any failure of an induction heater due to load variance thereof resulting from energizing the degaussing coil is considered.